Apple sold more iPads in 2013 than it sold Macs in the previous two decades -- combined. It's sold 210 million iPads since the tablet debuted in 2010 -- proving it's no fad. But iPad sales have flattened compared to the previous quarter, and analysts are asking whether we're approaching "peak iPad" -- when the market has saturated itself such that anyone who wants or needs an iPad now has one, and future sales will come from people replacing iPads they already own.

Former Apple executive -- and one of the smarter tech pundits -- Jean-Louis Gassée proposed recently the very notion: iPads have become like PCs in that the market will soon have no more room for growth from new users. As with PCs, Gassée suggests that iPads do what they do well, but what they do is no longer evolving. I can buy that argument -- it's been a while since I've seen a new app for the iPad that did something really new or different.

I've attributed that to a lack of imagination on the part of developers, all of whom seem to think adding a social facet to an app will make it interesting. (It doesn't.) And I believe there's been a slowdown in the innovation of the underlying hardware -- thinner, faster, lighter hits a point that doesn't matter much, and I think we're basically there now.

Apple's still the innovator in mobile, bringing us the iPhone and iPad themselves, then significant follow-on innovations like the Retina display in the iPhone 4, Siri in the iPhone 4S, and the fingerprint sensor, 64-bit A7 processor and 64-bit iOS 7, and the M8 motion coprocessor in the iPhone 5S. Its competitors have largely copied these innovations, though Google also pioneered voice-based assistance and Samsung reinvented the stylus in a compelling way in its Note series. (FYI: By "innovation," I do not mean invention. Innovation is the magic of making ideas work in surprisingly good, expectation-setting ways first.)

Still, notice my list of major recent mobile innovations from Apple. They all came to the iPhone first, with the iPad following on. In fact, the fingerprint sensor hasn't yet arrived on the iPad. Apple is not using the iPad as its innovation wedge, but as an innovation confirmer.

With the decreased pace of innovation by Apple and the ecosystem of app makers, a slowdown in the growth of iPad sales makes sense. Also, consider the fact that -- as with a PC -- you don't need to replace your iPad too often. There have been five iPad generations since 2010, and the truth is that the second-generation iPad 2 from 2011 is still a nice tablet for llght usage. If you have a 2012 third-gen iPad, a 2013 fourth-gen iPad, or a 2014 iPad Air, you have a very speedy, highly capable device that should be more than adequate for several more years of use.

You can see the effect of the slow-replacement cycle in Apple's earnings report this week. Two-thirds of iPad buyers in the last quarter didn't own an iPad before, whereas half of iPhone buyers didn't own an iPhone before. Yet iPad unit sales were down 16 percent versus a year ago and iPhone unit sales were up 16 percent versus a year ago. Those numbers indicate that existing iPad owners aren't buying new iPads, but existing iPhone owners are. (No, iPad owners aren't buying Android tablets instead; Android tablet sales remain anemic outside of the Kindle Fire e-reader .)

The lag in iPad innovation means a further lag in user upgrades and, thus, further flattening of Apple's iPad sales over the long term. (The iPhone has more room for growth, especially overseas, and a more frequent replacement cycle, but it is on the same trajectory longer-term.)